Sen— This image of Cornelia Crater on the giant asteroid Vesta was taken by NASA's Dawn mission from about 420 miles (680 kilometres) above the surface during a high-altitude mapping orbit. The dark material that speckles the surface of the asteroid is helping scientists to understand the impact environment early in Vesta's evolution. Scientists under the lead of the Max Planck Institute for Solar System Research (MPS) in Germany have shown that impacting small asteroids delivered dark, carbonaceous material to the protoplanet.
Vesta has a diameter of approximately 530 kilometers and is the one of the few protoplanets in our Solar System still intact today. As with other protoplanets, Vesta underwent complete melting approximately 4.5 billion years ago, but most of the volcanic activity on Vesta is thought to have ceased within a few million years, making it a time capsule from the early solar system.
"First, we created a map showing the distribution of dark material on Vesta using the framing camera data and found something remarkable," explains Dr. Lucille Le Corre from the MPS, one of the lead authors of the study.
The images were obtained during Dawn's year-long visit to Vesta that ended in September 2012. The carbon-rich material appears around the edges of two giant impact basins in Vesta's southern hemisphere. Analysis suggests that the dark material was delivered by the object that created the older of the two basins, known as Veneneia, about 2 to 3 billion years ago. Some of those materials were later covered up by the impact that created the younger basin, Rheasilvia. These impacting small asteroids altered Vesta's shape and its surface composition, delivering dark, carbonaceous material to the asteroid. It is suggested that similar events may have provided the inner planets like Earth with carbon, the essential building block for organic molecules. Color spectra of dark material on Vesta are identical to carbon-rich inclusions in HED meteorites - named for their mineral content (Howardite, Eucrite, Diogenite). "Our analysis of the dark material on Vesta and comparisons with laboratory studies of HED meteorites for the first time proves directly that these meteorites are fragments from Vesta" says Le Corre.
"The evidence suggests that the dark material on Vesta is rich in carbonaceous material and was brought there by collisions with smaller asteroids," explains Prof. Dr. Vishnu Reddy from the MPS and the University of North Dakota, the lead author of the paper. "We believe that the Veneneia basin was created by the first of two impacts two to three billion years ago." In fact, impact modeling presented in the paper reproduces the distribution of dark material from such a low velocity impact. The results were published in the latest issue of the journal Icarus.
Launched in 2007, NASA's Dawn's probe was launched to study Vesta and the dwarf planet Ceres with the aim of improving our understanding of the evolution of the early Solar System. Vesta, which orbits the Sun in the asteroid belt between Mars and Jupiter, was chosen as an example of a rocky world in the inner Solar System, whilst Ceres was chosen as an icy world whihc formed much further away from the Sun.
Dawn left Vesta in early September and is now on a long journey to Ceres which appears to have similarities to the large icy moons of the outer Solar System. Dawn will arrive at Ceres in 2015.
NASA's Dawn mission is managed by NASA's Jet Propulsion Laboratory (JPL), a division of the California Institute of Technology.